Gellan gum is a hydrophilic colloid produced by the fermentation of a bacterium, Sphingomonas paucimobilis, which is a strain screened and separated from the nature by a broad selection. Gellan gum has a lot of useful properties.
The basic structure of the molecule of gellan gum is a main chain, constituted by the repeating units of four sugars. The single sugars participating the formation of the repeating units include glucose, rhamnose and glucuronic acid. In the original form of gellan gum, i.e., high acyl gellan, two substituents, including acetyl groups and glyceryl groups, co-exist. The two kinds of groups are located on the same glucose residue. For each repeat unit, there is a glyceryl group; and for every two repeat units, there is an acetyl group. In the low acyl gellan gum, the acyl groups are removed completely. The acyl groups have a significant influence on the property of gel. High acyl-type gellan gum produces a gel having the property of softness, full of flexibility and not having brittleness, while the low acyl-type gellan gum produces a gel having the property of firmness, not having flexibility, but with brittleness. As to the marketed gellan products, gellan gum has two kinds: one is the low acyl gellan gum, i.e., the acyl groups in the main chain of the molecule of gellan gum were removed completely or partially; and the other is the natural gellan gum, i.e., high acyl gellan.
High acyl gellan can play a good role in gelation, shaping, fixation and film-forming. And high acyl gellan has the following characteristics: being a shear force reversible and thermo reversible gel, having an excellent emanation property of fragrances, being able to be used with other aqueous gels, having cooperatively with starch, having an extremely good shaping role, being an elastic gel, and being able to improve the flavors. Therefore, high acyl gellan has broad applications in pharmaceutical, chemical and food industries. High acyl gellan is a good substitute of carrageenan, agar, pectin and other hydrophilic colloids, and has a high demand in markets.
At present, the oversea production of high acyl gellan is only conducted by CP Kelco Inc., a USA entity, whose production method has not been reported in our country. In our country, the extraction method for high acyl gellan comprises the following steps: firstly sterilizing the bacterium by high temperature and then flocculently depositing by adding an alkali metal chloride and isopropanol (Zany et al, A method of post extraction of high acyl gellan, China Patent Application Publication No. CN1687437A). The method has the following defects:
1. The method uses calcium chloride, a new impurity, introduced during the flocculation; and
2. The high acyl gellan product obtained by using this method has a poor appearance, a low purity, low gel strength, and a poor solubility.